Castle?

http://www.europe-cities.com/en/592/italy/rome/attractions/castles/

http://www.ricasoli.it/ Brolio, on the border between the territories of Siena and Florence, soon became the stage for all the disputes of the period, representing the Florentine bulwark against the fearsome Siena. Through the centuries the castle has suffered attack and destruction in numerous historical battles, from Aragonese and Spanish attacks during the fifteenth century, to disputes in the seventeenth century right through to bombings and artillery attacks during the second world war. Stunning views of Tuscany from this castle set high atop a hill! It's also a winery for Chianti Classico.

"Set in stone." I get it! Anyway, check out Castel Sant'Angelo right in the heart of Rome. Very cool. Lots of tunnels and amazing views from the top, with a huge angel above you brandishing a sword. Here's a photo of the castle that I took this past May. And here's a short video clip from the top of the castle, taken the same day. In Florence, you should look at the Pitti Palace as a candidate. Happy travels.

Very near to Siena is an entire city that a 4-year-old can handle - Monteriggione. It's very small, completely surrounded by walls, some of which have ramparts you can walk. More ruins than castle, but a treat nonetheless, and you can walk the whole town in an hour or so.

Another vote for Monteriggioni: http://www.lodgephoto.com/galleries/italy-tuscany-chianti-monteriggioni/ They also have a medieval festival with costumes, reconstructed battles, jousting etc. at the end of June / beginning of July.

I would opt for Castel Sant'Angelo in Rome. It's in the center so you can easily visit without making an entire day of it. There are some great passageways and weapons your child will enjoy seeing - a catapult, cannon and cannonballs. Plus he can enter the prison cells.